The present invention relates to speed limiters for internal combustion engines, and more particularly to speed limiters for small internal combustion engines of the type used to power lawnmowers, snowblowers, generators and the like.
Conventional ignition apparatus for internal combustion engines comprises a primary and a secondary winding that are inductively coupled with one another, a spark plug connected across the terminals of the secondary winding, and control switch means for closing a circuit to enable current to flow in the primary winding and for opening that circuit at a time when the spark plug is to be fired. In battery ignition systems, the closing of the circuit that includes the control switch means allows battery current to flow in the primary winding. In a magneto ignition system, an electromagnetic field is induced in the primary winding by an orbitally moving magnet in cooperation with a fixed ferromagnetic core around which the primary and secondary windings are wound. The closing of the control switch means short-circuits the primary to allow current flow in it.
In either case, opening the primary circuit brings an abrupt change in a flux field with the secondary winding, thereby inducing a high voltage across the secondary and causing the spark plug to fire.
The conventional control switch means typically includes a pair of hard metal breaker points that are actuated by a mechanism having a cam that rotates in timed relation to the engine's cycle. More recently, the control switch means includes a semiconductor device such as a transistor, and a simple means for turning on and off the semiconductor device in timed relation to the engine's cycle.
It is often desirable to limit the speed of an engine to a predetermined maximum speed. Several types of electronic speed limiters are known. One type operates off the engine's alternator. Since the alternator typically provides a voltage proportional to the engine's speed, controlling the maximum voltage that may be reached by the alternator then controls the engine's maximum speed.
Electronic speed limiters are also known which are an integral part of the engine ignition system. Such speed limiters have the disadvantage that they cannot be easily retrofit onto an existing engine's ignition system without replacing at least a portion of the ignition system. Therefore, it is desirable to provide a simple engine speed limiter which may be retrofit onto an existing engine by connecting it to the engine's ignition system.